The principal thrust of the projects during the past year has been to determine mechanisms of transcapillary exchange of ions and molecules into bone. The indicator-dilution technique has allowed measurement of permeability ratios of Sr85, Fl, EHDP, antipyrine and sucrose. The permeability ratios are compared to ratios of free diffusion coefficients. The studies suggest that free diffusion is the principal mechanism for moving Sr85 across the bone capillary wall. Sr85 extraction (E) has been shown to be nearly independent of flow, F, so that Sr85 clearance (C) does provide a reliable index of flow. Utilization of fracture site in experimental ulnar fractures, while plates as a fixative showed more blood flow and more endosteal bone formation. Reinforcing plate fixation in experimental tibial fractures with methylmethacrylate profoundly decreases fracture site blood flow, and in fact whole tibial blood flow. Transcapillary permeability of Sr85 is not altered in hyperparathyroid dogs compared to control. Bibliographic references: Paradis, G.R. and Kelly, P.J.; Blood flow and mineral deposition in canine tibial fractures. J. Bone and Joint. Surg. 57A:220-226, 1975. Barron, S.E. and Kelly, P.J.: Effect of diffeent fracture treatments on blood flow and healing. J. Bone and Joint Surg. 57A:569, 1975.